To be honest, the easiest way is to just get another (cheap) Express and create a Canada account (when you get to Canada) to use on it. The other way is to create a Canada account when you're there, factory reset the Express, and link it to the Canada account and then perform the same operation when you get back (factory reset it and link it to the US account). You will have no end of problems if you try to use a device that was set up in one region and move to another region or link a device to an account that isn't associated with your current physical location/region. Channels that are only available in one region won't work even if you're physically in that region.
Having two accounts means you don't want to subscribe to anything through Roku since subscriptions are tied to the Roku account. Always subscribe directly with the provider when possible.
One other thing to note: Roku websites seem to rely heavily on cookies, so it's far too easy to create an account that isn't associated with the correct region even if you're physically in that region. If you already have a Roku account and you want to set up a new one in a different region make sure you clear your cookies before creating the account and make sure the region that's displayed in the bottom corner of the Channel Store website ( https://channelstore.roku.com ) shows the region you're expecting. Using a private browsing window or Incognito window is another way to avoid old stored browser information interfering with the process.
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